There are some hardware differences, the later consoles have slightly improved dvd drives and pcbs just to improve on reliability mainly but there are also anti-piracy measures added. Like anything when Nintendo got back faulty wii's they found out what was failing and improved it to increase reliability. However wii's are reliable consoles from day one. I don't think there has been any radical revision in hardware yet its not like the 360 or ps3 where the companies are desperate to reduce costs.

My own guess for the future is when the wii becomes a budget console near the end of its life they will lose the slot loading dvd drive and go for a top loading mechanism. The current dvd drive is meant to be more expensive than the pcb in the wii and also the least reliable part of the wii.

I've also heard the wii may get an increase in built in nand memory to something like 2 or 4gig to enable nintendo to sell more wiiware and virtual console games. Maybe this will come in with the new colour range of wiis that everyone is expecting at some point.